A conventional positioner is attached to a regulating valve provided in a pipe through which fluid flows so as to control the opening of the regulating valve.
This positioner includes a controlling portion obtaining the difference between a valve opening setting value sent from a higher-level device and an actual opening value fed back from the regulating valve and outputting, as a control output, an electric signal obtained by applying a predetermined calculation to the difference, an electric-pneumatic converter converting the control output from the controlling portion to an air pressure signal, and a pilot relay amplifying the air pressure signal converted by the electric-pneumatic converter and outputting the amplified signal to the setting/operating device of the regulating valve (see Japanese Unexamined Patent Application Publication No. S62-28118, for example).
FIG. 9 illustrates the structure of the main part of a conventional positioner. In this drawing, reference numeral 1 indicates a positioner, reference numeral 2 indicates a regulating valve, and the regulating valve 2 is provided with an opening sensor 3 detecting the opening (valve opening) of the regulating valve. The positioner 1 includes a controlling portion 11, an electric-pneumatic converter (EPM) 12, and a pilot relay 13. The valve opening of the regulating valve 2 detected by the opening sensor 3 is fed back to the controlling portion 11 as an actual opening value θpv.
In the positioner 1, the controlling portion 11 obtains the difference between a valve opening setting value θsp from a higher-level device (not illustrated) and the actual opening value θpv from the opening sensor 3 and outputs an electric signal obtained by applying PID control calculation to this difference as a control output MV.
The electric-pneumatic converter 12 converts the control output MV from the controlling portion 11 to an air pressure signal (nozzle back pressure) Pn. The pilot relay 13 amplifies the air pressure signal Pn from the electric-pneumatic converter 12 and outputs the amplified signal to a setting/operating device 2a of the regulating valve 2 as an air pressure Po. This causes air with the air pressure Po to flow into the diaphragm chamber of the setting/operating device 2a to adjust the opening of a valve 2b of the regulating valve 2.
In the positioner 1, the electric-pneumatic converter 12 and the pilot relay 13 constitute an electric-pneumatic converting portion 14 converting the control output MV from the controlling portion 11 to the air pressure (output air pressure) Po for the regulating valve 2. In addition, a supply air pressure (instrumentation air) Ps from the outside is supplied to the electric-pneumatic converter 12 and the pilot relay 13.
FIG. 10 illustrates the main part of the regulating valve 2. The regulating valve 2 is provided with a valve stem 2d moving up and down by the air pressure Po from the positioner 1 supplied to a diaphragm chamber 2c in the setting/operating device 2a and a packing gland 2f is provided between the outer peripheral surface of the valve stem 2d and the inner peripheral surface of a stem insertion hole 2e as illustrated in FIG. 10. The packing gland 2f includes a plurality of ring-shaped packings provided in close contact with each other in the shaft direction of the valve stem 2d so as to prevent fluid from leaking from the gap.
In the positioner 1, it is necessary to set appropriate control parameters matching the characteristics of the regulating valve 2 for the controlling portion 11 to properly control the opening of the regulating valve 2. For this purpose, before actually controlling the opening of the regulating valve 2 after newly installing (or replacing) the regulating valve 2 in the field or during execution of periodic maintenance, the control parameters used for the controlling portion 11 are tuned. The tuning of the control parameters is performed automatically during automatic setup or the like (see Japanese Patent No. 3511458, for example).
In this case, when receiving an automatic tuning instruction, the controlling portion 11 actually drives the regulating valve 2 and obtains, as the operation time, the response time when the valve opening of the regulating valve 2 continuously shifts from, for example, the 10% position to the 90% position and, based on the obtained operation time, determines the size of the setting/operating device 2a with reference to the setting/operating device size/operation time table defining the correspondence between the size of the setting/operating device and the operation time (step S101 illustrated in FIG. 11). The operation time in this case is generally referred to as the operation speed, but the operation time is used in this specification.
Then, the controlling portion 11 obtains, as the friction, the sliding resistance (stem reciprocating motion resistance) of the valve stem 2d of the regulating valve 2 based on a step response from, for example, the 40% position to the 60% position of the valve opening position of the regulating valve 2 (step S102) and determines the hysteresis level (H/M/L) based on the obtained friction with reference to a hysteresis level (HYS)/friction table (step S103).
Then, the controlling portion 11 selects the corresponding control parameters with reference to a control parameter table defining the correspondence among the size of the setting/operating device, the hysteresis level, and the control parameters based on the size of the setting/operating device determined in step S101 and the hysteresis level determined in step S103 (step S104) and sets the selected control parameters as the appropriate control parameters used to control the opening of the regulating valve 2 (step S105).
However, in the regulating valve in which the valve opening is controlled by such a positioner, the operation time of the setting/operating device changes depending on the supply air pressure (Ps) and the sliding resistance (friction) of the valve stem of the regulating valve. The operation time of the setting/operating device tends to become short as the supply air pressure increases and tends to become long as the friction increases (that is, the hysteresis increases). When the supply air pressure is high (>400 kPa) or the hysteresis is large (>10%), the effect on the operation time becomes large.
When the operation time of the setting/operating device changes due to the supply air pressure or friction, the operation time deviates from the range of the normal operation time in determination of the size of the setting/operating device in step S101 illustrated in FIG. 11, the size of the setting/operating device is incorrectly determined, and the control parameters selected in step S104 are not optimum control parameters, possibly causing problems (such as large stabilization time, large overshooting, and hunting) in the dynamic characteristics of the positioner.
Although the control parameters may be adjusted manually to address this problem, the user or the service department cannot easily address it since setting items are many and this work needs adjustment skills.
The invention addresses the above problem with an object of providing a positioner that can select and set appropriate control parameters even when the operation time of the setting/operating device changes due to the supply air pressure or friction.